1. Field of the Invention
This invention relates to an optimal orientation of berlinite (alpha-aluminum orthophosphate) for surface acoustic wave (SAW) device applications.
2. Description of the Prior Art
SAW technology for signal processing has reached maturity. A broad range of devices is now produced in substantial quantity for both military and commercial applications. These devices, for use in the VHF and UHF range, include filters, oscillators, delay lines, convolvers, and various types of sensors. Processes for preparing these devices are well-known in the art (see, e.g., H. I. Smith, "Surface Wave Device Fabrication," in Surface Wave Filters, edited by H. Matthews (Wiley, New York, 1977), pp. 165-217). The devices typically involve interdigitated electrodes (see, e.g., G. S. Kino et al., "Acoustic Surface Waves," Amer., October, 1972, p. 51)
Nearly all SAW devices currently are produced either on ST-cut quartz or lithium niobate substrates. ST-cut quartz has excellent thermal stability but a rather low piezoelectric coupling coefficient. Lithium niobate has strong piezoelectric coupling but poor temperature stability. A few SAW devices are made on lithium tantalate, which has moderate piezoelectric coupling and fair temperature stability.
All crystals that lack a center of inversion symmetry are piezoelectric. Very few of them have temperature compensated orientations, however, because that depends on their having one of the following anomalous properties: (1) a positive temperature coefficient of one or more elastic constants or (2) a negative coefficient of thermal expansion. Berlinite is isotructural with alpha-quartz, and, as is true with quartz, one of its elastic constants (C.sub.66) has a positive temperature coefficient. Barsch and Chang (IEEE Trans. Sonics Ultrason. SU-23, 127 (1976)) determined the berlinite equivalents of the temperature compensated quartz cut for bulk acoustic wave devices and also found that piezoelectric coupling of berlinite was several times larger than that of quartz. Subsequently, several researchers have predicted temperature compensated berlinite cut for SAW devices, using calculations based on Barsch and Chang's elastic, dielectric, and piezoelectric constants, and their temperature co-efficients. Among these predictions are X-axis boules cut at 80.4.degree. (U.S. Pat. No. 4,109,172, issued Aug. 22, 1978, to O'Connell); 87.1.degree. (Henaff et al., Ferroelectrics 42, 161 (1982)); and 92.75.degree. (Jhunjhunwala et al., J. Appl. Phys. 48, 887 (1977)).
A hydrothermal process and apparatus for growing berlinite crystals suitable for use in this invention were disclosed in U.S. Pat. No. 4,382,840, issued May 10, 1983, to chai et al.